In a technical field of a vehicle control, a by-wire system has been implemented. The by-wire system has a by-wire control circuit which electrically controls actuators driving components of a vehicle according to a command of a driver of the vehicle. For example, a shift-by-wire system is well known. The shift-by-wire system changes a shift gear of an automatic transmission according to a command of a driver. The shift-by-wire system has an electric actuator which drives a gear mechanism of the automatic transmission to change the shift gear. The electric actuator has a high-speed motor and a reduction gear which reduces the speed of the motor. In a case that a brushless motor, such as a switched reluctance (SR) motor, is employed as the high-speed motor, an incremental type encoder is generally provided to the actuator to output pulse signals according to a rotational position of the motor. The encoder realizes an optimal magnetization energization control of the motor.
In order to drive the gear mechanism of the automatic transmission accurately, it is necessary to detect an absolute rotational position of an output shaft of the reduction gear and to drive the actuator based on the detected absolute rotational position. A linear output sensor or an absolute type encoder is provided at a vicinity of the output shaft to detect the absolute rotational position. Alternatively, a neutral switch is provided on the output shaft. Also, an absolute type encoder which can detect a lot of rotations can be provided on the motor to detect the absolute rotational position of the output shaft.
However, a linear output sensor and an absolute type encoder have disadvantage in their reliability and durability. Even if the linear output sensor and the absolute type encoder can detect the rotational position of the output shaft, a rotational angle of the output shaft can not be detected. Thus, the linear output sensor and the absolute type encoder are not desirable for the shift-by-wire system in view of fail-safe. Further, the absolute type encoder has complicated structure and is expensive. Also, the linear output sensor is relatively expensive.
Japanese Patent No. 4248290 (US-2006-0207373A1), which will be referred to as Patent Document 1 hereinafter, shows a shift-by-wire system in which a shift gear is changed between a parking (P)-mode position and non-parking (non-P)-mode position. In this shift-by-wire system, only an incremental type encoder for magnetization energization control of the motor is employed to learn a reference position corresponding to an absolute position of the output shaft of the actuator. Based on the learned reference position and a predetermined rotational quantity, the absolute value of the output shaft is established and detected. By employing the increment type encoder to detect the absolute position of the output shaft, manufacturing cost of the system can be decreased.
Further, in the shift-by-wire system shown in Patent Document 1, when an electric power source of the vehicle is turned on, a reference position is learned, which corresponds to at least one of the P-mode position and the non-P-mode position. According to a vehicle condition, such as vehicle speed, of when the power source is turned on, it is determined which reference position of the P-mode position or the non-P-mode position is learned. Thereby, even when the electric power source is turned on immediately after the power source is momentarily turned off, the vehicle can be safely controlled under an ordinal control condition promptly.
However, in the shift-by-wire system shown in Patent Document 1, it is assumed that the automatic transmission has two mode positions (P-mode position and non-P-mode position). Thus, if this shift-by-wire system is applied to an automatic transmission having four mode positions, such as a parking (P)-mode position, a reverse (R)-mode position, a neutral (N)-mode position, and a drive (D)-mode position, it is likely that various problems will occur. For example, if the electric power source is momentarily turned off and then the power source is turned on when the actual mode position is the N-mode position, the mode position is brought into the D-mode position or the P-mode position by the learning control. Thus, the vehicle suddenly runs or stops, which may make the driver anxious. Also, if the electric power source is momentarily turned off and then the power source is turned on when the actual mode position is the R-mode position, the mode position is brought into the D-mode position or the P-mode position by the learning control. Thus, the vehicle suddenly runs forward or stops, which may make the driver anxious.